The present invention relates to carbon fiber paper and an electrode substrate for a fuel cell using the paper and more particularly an electrode substrate for a solid polymer electrolyte fuel cell and its production method.
As compared with an electrode for a phosphoric acid fuel cell, an electrode for a solid polymer electrolyte fuel cell is required to have gas diffusion-permeability, strength to be durable to handling, flexibility, strength to stand the compression at the time of electrode production and electrode assembly, and the like. Further, since the solid polymer electrolyte fuel cell is required to be small in size, as compared with a phosphoric acid fuel cell, the electrode for it is also required to be thin. As the electrode for such a solid polymer electrolyte fuel cell, mainly used are those which are produced by forming paper from carbon short fibers, impregnating the paper with a thermosetting resin, curing the resin, and then carbonizing the resultant paper, and in order to improve the productivity of a fuel cell, the electrode has to be flexible enough to be rolled in a roll. However, many electrodes ever made available are thick and mostly easily broken when being bent. Further, since those conventional electrodes have a few contacting points of carbon fibers, they have a problem that the conductivity becomes worse if the porosity is increased.
A porous carbon electrode substrate provided with an improved conductivity by mixing carbonaceous milled fibers is described in Japanese Patent Laid-Open No. 142068/1995, however the substrate is too thick in thickness and thus insufficient in flexibility to be used for a solid polymer electrolyte fuel cell.
Invention of a porous carbon plate and its production method is described in Japanese Patent Laid-Open No. 157052/1997, the electrode of the invention has a low bulk density and therefore it cannot be said that the conductivity of the electrode is sufficient.
Objects of the present invention are to solve the above described problems and provide an electrode substrate for a fuel cell having high conductivity and flexibility, its production method, and carbon fiber paper suitable for producing the electrode substrate.
The present invention provides carbon fiber paper consisting of an organic high-molecular compound as a binder and carbon fibers, the carbon fibers containing thin fibers with an average diameter smaller than 5 xcexcm and an average fiber length of 3 to 10 mm.
In the carbon fiber paper of the present invention, the organic high-molecular compound is preferably polyvinyl alcohol, and the organic high-molecular compound is preferably pulp-like substances or short fibers of an acrylonitrile type polymer. Further, the foregoing carbon fibers are preferably only of polyacrylonitrile type carbon fibers. Furthermore, the foregoing carbon fibers are preferably a mixture of thin fibers with an average diameter larger than 3 xcexcm and smaller than 5 xcexcm and an average fiber length of 3 to 10 mm and thick fibers with an average diameter not smaller than 5 xcexcm and smaller than 9 xcexcm and an average fiber length of 3 to 10 mm. It is also preferable for the foregoing carbon fibers to contain not less than 40% by mass of the foregoing thin fibers.
The present invention also provides a porous carbon electrode substrate for a fuel cell having a thickness of 0.05 to 0.5 mm and abulk density of 0.3 to 0.8 g/cm3, and also having a bending strength of 10 MPa or higher and a deflection of 1.5 mm or more at the time of bending, measured by a three-point bending test in conditions of using a sample of 1 cm width, a strain rate of 10 mm/min and a distance between supporting points of 2 cm.
The electrode substrate of the present invention preferably has a length of 1 m or larger and can be rolled around a roll with an outer diameter of 50 cm or smaller. Further, it is preferred that the electrode substrate contains carbon fibers, and the foregoing carbon fibers are only of polyacrylonitrile type carbon fibers. Furthermore, it is preferred that the electrode substrate contains carbon fibers, and the carbon fibers are a mixture of thin fibers with an average diameter larger than 3 xcexcm and smaller than 5 xcexcm and an average fiber length of 3 to 10 mm and thick fibers with an average diameter not smaller than 5 xcexcm and smaller than 9 xcexcm and an average fiber length of 3 to 10 mm. It is also preferable for the electrode substrate to contain carbon fibers which contain not less than 40% by mass of the foregoing thin fibers in the total carbon fibers.
The present invention also provides a method of producing a porous carbon electrode substrate for a fuel cell by impregnating carbon fiber paper consisting of an organic high-molecular compound as a binder and carbon fibers with an average diameter smaller than 5 xcexcm and an average fiber length of 3 to 10 mm with a thermosetting resin, curing the thermosetting resin by heating and pressing treatment, and then carbonizing the resultant paper.
In the production method of the present invention, it is preferable to use carbon fiber paper, among the above described carbon fiber paper, wherein the carbon fibers are made of a mixture of thin fibers with an average diameter larger than 3 xcexcm and smaller than 5 xcexcm and an average fiber length of 3 to 10 mm and thick fibers with an average diameter not smaller than 5 xcexcm and smaller than 9 xcexcm and an average fiber length of 3 to 10 mm. Further, the foregoing heating and pressing treatment is preferably carried out continuously in the whole length of the carbon fiber paper. Further, prior to the heating and pressing treatment, the carbon fiber paper impregnated with the thermosetting resin is preferably pre-heated. Furthermore, the heating and pressing treatment is preferably carried out using a continuous type hot press machine equipped with a pair of endless belts, or the heating and pressing treatment is preferably carried out using a continuous type hot roll press machine. In the heating and pressing treatment, the pressure application is preferable to be carried out at a line pressure of 1.5xc3x97104 to 1xc3x97105 N/m.
In the production method of the present invention, the foregoing carbonization is preferably carried out continuously in the whole length of the carbon fiber paper. Further, the electrode substrate obtained by the foregoing carbonization is preferable to be rolled around a roll with the outer diameter of 50 cm or smaller. Furthermore, a conductive substance is preferably added to the foregoing thermosetting resin.